1. Field of the Invention
The present invention relates to a screw method executed by a numerical control apparatus, and more specifically, to a screw method for screwing a pipe having threads with a different lead on the outside and inside thereof into an object and assembling the same thereto.
2. Description of the Related Art
When a device is assembled, there is a case in which a pipe having threads with a different pitch on the outside and inside thereof must be assembled to an object. In this case, a method making use of a rigid tapping cycle and a rigid reverse taping cycle performed by a numerical control (CNC) machine tool may be employed. For example, the inside of the pipe is screwed into the male thread of a spindle of the CNC machine tool and first the outside of the pipe is screwed into the object by controlling the operation of the spindle and a feed axis by the rigid tapping cycle. After the completion of the screw operation, the spindle is relieved from the inside of the pipe by controlling the operation of the spindle and feed axis by the rigid reverse tapping cycle. Note, the rigid tapping cycle and rigid reverse tapping cycle refer to a tap machining control by which the rotation of the spindle is synchronized with the feed effected by the feed axis at all times by interpolating the spindle and feed axis.
In the rigid tapping cycle and rigid reverse tapping cycle of the CNC machine tool, an amount of feed of the feed axis per one revolution of the spindle is set equal to the pitch (lead) of the screw of a tap. As a result, the feed speed in the rigid tapping cycle (in a screw operation) is equal to the feed speed in the rigid reverse tapping cycle (in a relief operation). Consequently, when a pipe is screwed into an object and assembled thereto, if the outside and inside of the pipe have the same thread pitch, the rigid tapping cycle and rigid reverse tapping cycle of a CNC machine tool can be effectively used.
Nevertheless, when the outside and inside of the pipe have a different thread pitch, a feed speed must be changed in accordance with the pitch in a screw operation and relief operation, and thus the rigid tapping cycle and rigid reverse tapping cycle with the same feed speed as described above cannot be used. Therefore, when a pipe having threads with a different pitch on the outside and inside thereof is screwed into an object and assembled thereto, a CNC machine tool cannot be used and a dedicated machine must be used. Consequently, an operation process cannot be automatically carried out and thus an operation efficiency is deteriorated.